The invention relates in general to cutting knives and methods, and notably to the field of cutting knives and method for webs, and particularly plastic webs such as polyester, polyethylene, PET (polyethylene terephthalate) or PEN (polyethylene naphthalate) webs.
The manufacture of thin webs of plastic material such as polyester involves different operations to be performed on the webs, such as stretching of the webs, winding up of the webs on rolls, etc. It is also necessary to cut the webs, e.g. after a roll of a given size has been wound up. This causes several problems. The first problem is the actual cutting of the webs. The second problem is to deal with the leading edge and the trailing edge created through the cutting of the web. The third problem is to protect the core around which the web is supposed to be wound, since this core is supposed to be used several times. Ideally, a good cutting solution for webs and particularly plastic webs, should provide a clear and accurate cutting, and should allow the leading edge of the web after the web to be wound up easily, without stopping the web or slowing the speed of the web, and without any crease or fold in the web.
One type of solution for cutting such a web, while it is being wound up, it to use a knife moving transverse of the web winding direction or longitudinal axis of the web, that is along the axial direction of the rolls. Such a cutting apparatus is for instance described in U.S. Pat. No. 4,637,567, or in EP-A-0 606 662. The first of these documents discloses a knife formed of a flat blade having a flat surface, that moves transverse of the web longitudinal direction. The flat surface of the knife is brought into contact with the surface of the roll on which the web is to be wound; this contact avoids any crease or fold in the web as it is being cut. In addition, to ensure that the web fits on the new roll, the web may be moistened before it reaches the roll. EP-A-0 606 662 discloses a rotary cutting knife that travels along the axial direction of the roll.
For such knives moving along the axial direction of the rolls, the leading and trailing edges of the web are generally not perpendicular to the longitudinal axis of the web. Since the web to be cut travels along its winding direction, with a speed that can reach 450 m/min (7,5 m/s), the usual angle between the web edge and the longitudinal axis of the web may be quite high, even if the knife moves at a comparable speed. EP-A-0 606 662 exemplifies a knife moving at a speed of 600 m/min, and an angle of the leading edge of the web around 40xc2x0. This causes loss of web at the beginning and end of each roll, or more generally, each time the web is cut. The amount of web lost may be up to several percents. It also makes the handling of the web more difficult for subsequent operations, due to the triangular shape of the end of the web, and thus the final conical shape of the wound up roll. Moreover, the winding up of the web on the winding roll is made more difficult since the triangle-shaped leading edge causes an asymmetrical profile on the core of the roll. Finally, such knives have to move at high speed, and should be accelerated and decelerated over short distances, so as to limit the bulkiness of the apparatus. This creates mechanical problems.
Another type of solution uses a knife with a cutting edge extending transverse of the web, that is reciprocated along the web winding direction to cut the web. U.S. Pat.No. 5,464,166 discloses a rotary cut-off knife assembly, having a cylindrical cut-off roll with a knife along one generating line of the roll; the knife may project out of the roll or be retracted. The operation of this device is the following: the cut-off roll is brought into contact with the web, with the knife retracted in the roll, and the knife is projected outside of the roll when the web is to be cut. In this document, it is suggested that the linear speed of the cut-off roll is higher than the travel speed of the web, and is typically 25% greater than the speed of the web. In order to bring the leading edge of the web to the core of the new roll, this document suggest cutting the web in the neighbourhood of the core roll, while generating an electrostatic charge on the core roll. For this purpose, it is suggested that the gap between the tip of the knife and the core roll be about 1.5 mm when the knife severs the web. The solution suggested in this document ensures that the web is cut transverse to the winding direction. However, accelerating the cut-off roll at a speed higher than the one of the web may cause a problem, especially for very thin webs wound at high speeds; indeed, before the web is severed, the cut-off roll is in contact with the web and rotates at a higher speed. Moreover, there is a wedge of air between the web and the core roll at the time the web is cut. The presence of air creates creases or folds and makes it difficult to ensure that the leading edge of the web is smoothly and evenly placed on the core. Finally, it would be advantageous to avoid using an electrostatic charge system.
U.S. Pat. No. 4,789,109 uses a blade powered by a compressed air box for cutting a web transverse its winding direction. In this document, it is suggested to incorporate the blade and its air box to a pusher device for rotating a completely wound web out of the winding device. The winding of the web must however be stopped for cutting the web and getting the wound roll out of the winding apparatus. The new core roll is provided with a glue stripe or with a 2-sided tape, and is lowered over the leading edge of the web. This device is highly disadvantageous in that it implies stopping web supply each time a new roll must be started.
Other types of cutting apparatuses have been suggested; U.S. Pat. No. 5,335,869 discloses a blade entering into a recess of a roll; U.S. Pat. No. 5,285,977 discloses a similar system where projections of one roll enter into recesses on another roll. These systems cannot cut the web directly in the neighbourhood of the core of a roll, and propose complicated solutions for bringing the leading edge of the web to the core of the roll where it should be wound. U.S. Pat. No. 3,047,248 discloses a reciprocating knife, mounted on oil pressure actuators, in a cloth winding circuit.
To summarise, none of the various solutions of the prior art provides a cutting apparatus that may be used for the cutting of thin and ultra thin webs in a winding station, while ensuring:
a clean cut transverse the winding direction of the web;
a smooth and even placing of the leading edge of the web on a new core roll, without any need to stop supplying the web to the station.
Accordingly, the purpose of the invention is to provide a solution that overcomes the problems of the prior art. According to the invention, there is provided a high speed knife moving along the web direction, tangent to the web and adjacent the core of the new roll, so as to ensure a clean and clear cut of the web, and at the same time, a smooth and even placing of the leading edge on the core roll.
The invention proposes a cutting assembly, comprising an elongate housing at least two guiding shafts transverse to the housing and slidingly supporting a blade through at least one carriage; said at least one carriage being movable between a rearward position and a forward position; actuating means for displacing said at least one carriage from the rearward position to the forward position; holding means for holding said at least one carriage in its rearward position and for releasing said at least one carriage from its rearward position to the forward position. Prefereably, the blade is a flexible blade.
The actuating means may be operated by a pressurised fluid, preferably pressurised air. They may comprise at least one cylinder with a piston connected to one carriage. The holding means advantageously comprise at least one trigger engaging a protruding section of said at least one carriage, and releasing means for disengaging said trigger from said protruding section. The releasing means may also be operated by a pressurised fluid, preferably pressurised air. They preferably comprise an inflatable tube capable of displacing said trigger when inflated.
The assembly may also comprise return means for bringing said at least one carriage from its forward position to its rearward position, or elastic means for receiving said at least one carriage when it reaches its forward position.
Prefereably, the cutting assembly further comprises a nose part mounted on said housing, near to said forward position. The nose part may be an elastic part. The assembly may also comprise a flexible plate mounted on the lower side of said housing, near to said forward position, and adapted to tangentially receive the blade when the blade is in its forward position.
The invention also relates to a web winding apparatus, comprising at least a roll and a cutting assembly located near a roll, said cutting assembly having a blade movable between a forward position and a backward position, wherein, in the forward position of the blade, a cutting edge of the blade is substantially parallel to an axis of the roll while the blade is substantially tangent to a surface of the roll.
Preferably, the apparatus further comprises a transfer roll, the roll is a winding roll, and the cutting edge of the blade in its forward position lies near the nip point between the winding roll and the transfer roll. The angle between the blade and a plane tangent to the roll at the contact line of the blade with the roll is preferably between 0xc2x0 and 10xc2x0.
The invention finally relates to a process for cutting a thin web in a thin web winding station, comprising a roll receiving the web; a cutting assembly having a blade with a cutting edge, said blade being movable between a rearward position and a forward position where the cutting edge is substantially parallel to an axis of the roll and where the blade is substantially tangent to a surface of the roll, the process comprising the step of moving said blade from its rearward position so that its speed at the time it engages with said web is at least substantially equal to the speed of the web.
Preferably, the process comprises the step of further accelerating said blade after it engages with said web. The cutting assembly is advantageously at a location near the nip point between the transfer roll and the winding roll.